In some engineering applications structural components are affected by the fluids with which they are in contact. Examples include chemical process equipment and piping systems, water treatment and distribution systems, and oil and gas pipelines. In many of these applications, it is advantageous to monitor damage accumulation, predict component life, and control fluid properties to minimize damage to system structural components. Damage to structural components from fluids can include corrosion, erosion, scale deposition, oxidation or other chemical effects. Many structural components can be difficult to inspect, can be hidden from observation, can cause health and environmental damage in the event of failure, and/or can be costly to maintain. Advanced sensors are needed to actively monitor the physical effects of fluids in contact with structural components and minimize their effect. For example, such sensors can be used to provide feedback in control systems for the injection of green treatment chemicals and corrosion inhibitors to control corrosion, biological growth, scaling in water treatment, chemical process and boiler systems. Advanced monitoring of harmful physical and chemical effects will result in reduced maintenance costs, increased component service life, and safer operations.
Among various types of systems that employ physical contact between the fluid and the structural component, it is particularly desirable to be able to monitor physical and chemical processes when the fluid flows through a conduit. In such systems, the flow may influence erosion, corrosion and/or scale deposition of the conduit surface, potentially leading to breaches of the conduit by the fluid or to clogging the conduit. Therefore, it is important to be able to measure the metal loss or mass deposition of the conduit surface, especially at conduit bends where the metal loss or deposition rate is greatest. A limited range of measurement technologies are conventionally utilized to determine the rate and type of effects that damage the component. These technologies can be grouped into three general categories: 1) metal loss methods, 2) electrochemical methods, 3) acoustic methods.
Metal loss measurement methods include electrical resistance devices and mass loss coupons. An electrical resistance probe can continuously monitor cumulative corrosion rate of metal elements. Such a probe has a sample element that is exposed to the fluid flow. Physical and chemical interaction between the sample element and the fluid change the thickness and hence the electrical resistance of the sample element. The service life of the resistance probes is directly proportional to the probe thickness while the resolution is inversely proportional to it, with the highest resolution achieved at the expense of sensor life. The more sensitive resistance probes have response times of about 100 hours for corrosion rates of 1 MPY. Resolution is reduced by thermoelectric voltages and electromagnetic noise. Resistance probes provide for continuous monitoring without process interruption in many fluids, except highly conductive environments such as molten metal or conductive molten salts. Prior art electrical resistance probes are described, for example, in U.S. Pat. No. 6,693,445, or U.S. Pat. No. 6,946,855.
The mass loss coupons can be made of an alloy that is the same as the structural component being monitored or can be a standard material including steels, stainless steel, copper and brass. They are inserted into the process stream for a predetermined period of time, after which they are retrieved cleaned and weighed. Mass loss coupons are considered to be reliable for measuring corrosion over longer time periods at discrete intervals. They can be used in nearly any process, but do not allow for real-time monitoring, are labor intensive and require process interruption or significant space within the process system or structure being monitored. A prior art mass coupon corrosion rate monitoring system is described in U.S. Pat. Pub. No. 201110066388.
Electrochemical methods, including Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (EN) are used to monitor corrosion. These measurement techniques are used to quantify kinetics of electrochemical reactions associated with corrosion. Application is limited to conductive solutions and performance is restricted in non-aqueous environments. Like resistance sensors, the resolution of electrochemical methods is reduced by thermal and electromagnetic noise. Unlike resistance and mass loss coupon methods, electrochemical techniques are unable to detect metal loss due to erosion or to provide a direct measure of cumulative material loss.
A variety of acoustic methods have been described for sensing the metal loss or formation of scale in producing hydrocarbon wells and similar environments. Several methods are based on the measuring of the measurement of transit time of the acoustic waves propagating through the fluid, structural element or both (U.S. Pat. Nos. 4,669,310; 4,872,347; 5,072,388), or on the measuring of the attenuation of acoustic energy by the structural element (U.S. Pat. Nos. 5,092,176; 5,661,233). These methods generally have poor spatial resolution and sensitivity.
More sensitive acoustic resonance methods are known. In these approaches, either piezoelectric or mechanical resonator systems are externally excited and the changes in resonance frequency of the resonator are related to the change in mass of the material removed or deposited on the resonator surface. For example, an application of an on-line quartz crystal microbalance to monitor and control the formation of organic and inorganic precipitates from hydrocarbons and water has been described in U.S. Pat. No. 5,734,098. In this approach, resonant frequency changes are measured that were related to mass loss or deposition of the resonator surface. Piezoelectric acoustic resonators, while providing high sensitivity (detection of a thickness change of the order of 1 micrometer has been reported) do not allow a user to simply differentiate between the effects caused by changes in the mass of material deposited from a liquid and changes in the properties of the liquid (temperature, pressure, density and viscosity).
A simpler mechanical resonator system that makes use of a tuning fork or similar resonator for measuring the mass loss or deposition of scale in a surface process system has been disclosed, for example in U.S. Pat. No. 5,969,235; or U.S. Pat. Nos. 7,721,605; 7,681,449; 7,866,211. A in a piezoelectric resonator, an actuator is used to excite natural vibrating frequency of the resonator. The mass loss or accumulation of scale on the tines of the tuning fork causes a shift in the natural oscillating frequency of the tuning fork as measured by exciting the force by a suitable device, such as a piezoelectric cell. Typically the frequency response for these sensors is described by theoretical relations derived for a lightly damped harmonic oscillator with single degree of freedom. For a free oscillation, or if the forcing function is sinusoidal, the resonance frequency, f0, and quality factor, Q (a measure of the system damping and energy dissipation), can be represented by:f0=(1/2π)√(k/m)  Equation 1Q=(1/c)√(k*m)  Equation 2where m=system mass, k=system stiffness, and c=velocity dependent damping. Tuning fork sensors are widely used due to low cost and the simplicity of the sensor. There are, however, several fundamental problems that impede these devices from correctly measuring the effects of mass loss and/or scale deposition.
The first of these problems occurs during operation. More specifically, the forks should be positioned inside the conduit with the sensitive to corrodible surface of the tines in normal to the flow direction, while the wall of the conduit is parallel to the flow. The rate of the physical and/or chemical processes that lead to the mass loss and/or scale deposition are therefore different for the surface of wall and for the surface of the tines, respectively.
Second, the sensor, being inserted into the flow, disturbs the flow and may form complex turbulent flow patterns between the tines thus changing the mass loss and/or scale deposition rates as compared to interaction between the wall and the undisturbed flow.
Third, the connection between the resonance frequency and mass loss is not trivial, because the mass loss or mass gain along the tines of the fork affects both system mass (m) and system stiffness (k). Moreover, there is apparent confusion in the art about this connection. For example, U.S. Pat. No. 6,928,877 and U.S. application 2006/0037399 both employ resonators and teach a relationship between the resonance frequency and mass change that is consistent with the well-known relations cited above for a single degree of freedom lightly damped mechanical oscillator: a mass decrease will result in a frequency increase and a mass increase will result in a frequency decrease. U.S. Pat. No. 7,681,449 teaches away from the prior art by discovering that mass decrease from corrosion/erosion can also result in a resonance frequency decrease. U.S. Pat. No. 7,681,449 provides evidence that the stiffness of the resonator device is also governed by the system mass, and that relationship between system mass and stiffness is location dependent: the amount of change to the system stiffness is dependent upon where the mass is lost (or gained). U.S. Pat. No. 7,681,449 shows that by selecting the proper location on the vibrating element, it is possible that the change in the stiffness to mass ratio of Equation 1 can be stiffness dominated even though mass is being lost. In that case, a loss of mass will result in a frequency decrease, teaching away from the prior art. U.S. Pat. No. 6,928,877 also teaches to make the mass additions or losses at the tip of the resonator. For U.S. Pat. No. 7,681,449, the resonator's mass change location is designed to be close to the attachment point of the tine. At this location, mass loss has a sufficient impact on system stiffness as to cause a resonance frequency decrease.
In order to resolve this apparent controversy, recent systems (such as that disclosed in U.S. Pat. No. 7,681,449) employ multiple sensors with different corrodible characteristics, measure both resonance frequency fo and quality factor Q for each sensor, and refer to the mass loss rate via a complex relationship between measured parameters in attempt to compensate the inherent trade-off between the mass and stiffness change in the tuning fork mechanical resonance method.
Furthermore, mechanical response systems installed along a conduit wall, such as U.S. Pat. Pub. 2008/0141780, disclose a method in which a change in the mechanical response of a diaphragm when actuated is used to monitor the total change in diaphragm thickness and the rate of change in thickness so as to sense the physical effects of a fluid in contact with the diaphragm. In this approach, the diaphragm can be installed flush with the conduit wall with one surface of the diaphragm exposed to the flow. However, as is the case for tuning fork sensors, the corrodible surface and elastic element in this approach are combined in one element that complicates establishing the relationship between the mechanical property of the element and the effect of the flow. For a diaphragm to be sensitive to mass loss, it must be thin and thus prone to failure due to unexpected pressure changes. In addition, diaphragm mechanical characteristics (such as resonance frequency) are affected by the fluid pressure. To compensate the fluid pressure and temperature effect, U.S. Pat. App. 2008/0141780 makes use of a second reference probe with the diaphragm impervious to the fluid. This complicates the sensor and introduces additional risk of diaphragm failure. The present invention proposes a design that is more robust and does not require a reference probe for reliable measurements of the rate of physical and chemical effects of the flow on the sensitive surface.
Moreover, U.S. Pat. No. 7,770,463 and U.S. Pat. App. Pub. 2010/0326200 disclose a sensor for measurement of the shear force exerted by fluid on a floating element installed flush with the wall. This prior art does not teach of a relation between the mass loss of the element and the mechanical resonance of the assembly that comprises the floating element. Nor does it teach what part of the total surface of the mechanical assembly should be exposed to fluid influence and which should be protected to provide a reliable relationship between the effect of interest and mass loss or gain.
There is a need for a device and method for sensing techniques that may be used for the continuous on-line detection of fluid effects on structures, that addresses present challenges and characteristics such as those discussed above.